Thiamine deficiency is not uncommon in certain populations and clinical disease states such as Wernicke's encephalopathy or beriberi. Rapid parenteral repletion may be required, yet questions about the safety of IV thiamine have been raised because of reports of anaphylaxis. Our study was a prospective evaluation of the safety of thiamine hydrochloride given as a 100-mg IV bolus in 989 consecutive patients (1,070 doses). A total of 12 adverse reactions (1.1%) were reported. Minor reactions consisting of transient local irritation were seen in 11 patients (1.02%), and there was only one major reaction (0.093%) consisting of generalized pruritus. Thiamine hydrochloride may be administered intravenously without undue concern. Intradermal test doses before administration are not warranted unless patients have had previous allergic reactions.